Organic Rankine cycle and waste heat recovery

The high temperature Rankine steam cycle is well known in the power generation field. Increasing demand for energy efficiency has led to interest in heat recovery systems and other low heat sources, which created the need for a device which could convert surplus and low temperature heat into electricity. The generation of electricity from thermal sources at low temperature has become possible through the use of organic fluids, driving turbines and other expanders on the same principle as the steam cycle.

The name “Organic Rankine Cycle” (ORC) has been adopted to describe this energy extraction cycle. The fluids used are more efficient when used with heat at low temperature (below 300°C) and for low power applications (from a few kW to several MW).

The ORC is well suited for these applications, mainly because of its ability to recover low-grade heat and the possibility to be implemented in decentralised lower-capacity power plants. Steam cycles need high temperature, high pressures, and therefore high installed power in order to be profitable. The ORC offers a cost effective system for low power, lower temperature heat source applications where a steam plant would be expensive.

Conceptually, the ORC is similar to the steam Rankine cycle in that it is based on the vaporisation of a high pressure liquid which is in turn expanded to a lower pressure to release mechanical work. The cycle is closed by condensing the low pressure vapour and pumping it back to high pressure. The ORC involves the same components as a conventional steam power plant (a boiler, a work-producing expansion device, a condenser and a pump). However, the working fluid is characterised by a lower boiling temperature than water.

The low molecular weight of water requires the use of multistage expanders to obtain high cycle efficiency. A common feature of all organic working fluids used in ORC technologies is their high molecular weight and low boiling point. They also have critical temperatures and pressures far lower than water. For ORC engines with maximum temperatures below 200°C, fluids with higher molecular weights than water can provide high cycle efficiencies in less complex and less costly single stage expanders [1].

The ORC can have a beneficial effect on the energy intensity of industrial processes, mainly by recovering waste heat (i.e. heat that is otherwise lost). Installing an ORC to convert waste heat into electricity enables a better use of the primary energy. This approach is known as combined heat and power generation (CHP) through a “bottoming” cycle.

The ORC can have a positive effect on building energy consumption by using CHP systems. Fossil fuels are able to generate high temperature levels, and the ORC can take advantage of this high temperature to produce electricity, while the low level heat rejected by the ORC is still able to meet the needs of the building. This approach is known as combined heat and power generation through “topping” cycles.

The ORC can be used to convert renewable heat sources into electricity. This mainly includes geothermal, biomass and solar (CSP) sources. The ORC does not require multistage turbines and generally single stage turbines or other expansion devices such as screw or scroll expanders are used to generate energy. The ORC is well suited to trough type solar CSP systems, which traditionally use steam turbines to generate electricity, but which produce heat in the temperature range where the ORC is more efficient than the steam cycle.

The Rankine cycle

The simple Rankine cycle is an idealised closed vapour cycle. It consists of four stages as shown in Fig. 1.

Fig. 1: Simple Rankine cycle system.

The Rankine cycle is best described by means of thermodynamic curves, typically the vapour saturation curve and the T-S or P-h curves. An example of a Rankine cycle using a P-h curve is shown in Fig. 2.

The simple Rankine cycle consists of four processes or stages (Fig. 2):

Process 1 – 2: The working fluid is pumped from low to high pressure. As the fluid is a liquid at this stage, the pump requires little input energy.

Process 2 – 3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapour.

Process 3 – 4: The dry saturated vapour expands through an expander generating power. This decreases the temperature and pressure of the vapour, and some condensation may occur.

Process 4 – 1: The wet vapour then enters a condenser where it is condensed at a constant pressure to become a saturated liquid.

Fig. 2: Simple Rankine cycle [6].

More complex rankine cycles may involve several stages of preheating, superheating and reheating. Superheating is used to ensure that the vapour remains in a dry saturated vapour state during expansion, and that formation of droplets which damage turbine or expander components does not occur.

The first three properties allow the use of low temperature sources, the fourth allows expansion from a non-superheated vapour point without entering in the vapour area.

Fig. 3: Typical non-superheated ORC [3].

This aspect is fundamental, because is not necessary to superheat the fluid as at the end of the expansion the fluid is still in the vapour phase, avoiding any problems in the expander, especially if it is a turbine. In an ORC design is the choice of the fluid is based on the heat source.

The Rankine cycle uses three different types of fluids (Fig. 4) characterised by the slope of the T-s curve in the saturated vapour region.

Wet fluid has a negative curve in this region, and requires superheating to ensure that the vapour remains in a dry saturated vapour state during expansion. Typical liquids would be water and carbon dioxide. Wet liquids require superheating to avoid droplet formation during expansion.

Dry fluid has a positive curve in this region and will remain in the dry saturated vapour state during expansion.

Anisotropic fluid has an infinite curve (vertical) and represents the ideal liquid for the ORC.

Fig. 4: Types of fluid used in ORC systems [2].

The ORC cycle normally uses dry or anisotropic fluids.

The advantage of dry and isentropic fluids is that superheating is not required as the liquid remains in a saturated vapour state during expansion. Dry liquids give a higher power output for a given temperature of operation. Choice of the fluid for ORC operation will depend on the temperature of the heat source available. A range of fluids is available to accommodate a range of temperatures.

ORC fluids

ORC systems operate in the temperature range of 100 – 300°C for heat sources. Originally refrigerants were used as the working fluids, but these were useful for low temperatures only, and a new range of organic fluids has been developed to allow the full range of heat sources to be utilised. Table 1 lists several of the common fluids in use and their properties.

Fig. 5: T-s diagram for water and organic working fluids [5].

Commonly used ORC work fluids:

Toluene

(Cyclo)-pentane

Ammonia

Butane

Refrigerants

(R245fa)

Solkatherm

Siloxanes (silicone oils)

Silicone oils are commonly used in commercially available systems.

Table 1 lists some of the properties of fluids used in ORC systems.

Table 1: Properties of ORC fluids [2].

TCrit

PCrit

Boiling point

Eevap (1 bar)

Fluid

Formula/name

(ºC)

( bar)

(ºC)

(kj/kg)

Water

H2O

373,9

220,6

100,0

2257,5

Toluene

C7H8

318,7

41,1

110,7

365,0

R245fa

C3H3F5

154,1

36,4

14,8

195,6

n-pentane

C5H12

196,6

33,7

36,2

361,8

Cyclopentane

C5H10

238,6

45,1

49,4

391,7

Solkatherm

Solkatherm

177,6

28,5

35,5

138,1

OMTS

MDM

291.0

14,2

152,7

153,0

HMDS

MM

245,5

19,5

100,4

195,8

The saturation vapour curve of a typical silcone oil is shown in Fig. 6.

Fig. 6: Saturation vapour curve of MM silcone oil [2].

The simple ORC for a system using a dry fluid without superheating is illustrated in the T-s diagram of Fig. 7.

Fig. 7: Simple ORC using a dry fluid without superheating [1].

Advanced ORC systems

Advanced systems make use of additional preheat and superheat cycles to improve efficiency. A typical commercial system is the Siemens ORC module, illustrated in Fig. 8.

Fig. 8: Advanced multi stage ORC system [4].

Limitations

The limitation on the use of organic fluids is the maximum operating temperature of the working fluid. For heat sources <400°C the ORC is capable of achieving higher efficiency than a steam cycle. For temperatures above 400°C steam is more efficient [2].

Turbines and expanders for ORC systems

The thermodynamic characteristics of the fluids used make multistage axial flow turbines unsuitable for use with ORC systems. Turbines are usually single stage although some manufactures offer multistage design for specific fluids.

There are three systems commonly used with ORC

Radial outflow turbines

Screw expanders

Scroll expanders

Radial outflow turbines

50 kW upwards

In a radial outflow turbine or expander, vapour enters at the centre of the rotor and expands towards the outside, exiting at the perimeter of the rotor. The rotation speed is much lower than axial expansion turbines and generally a single rotor disc is used. Fig. 9 shows a typical example.

Fig. 9: Radial outflow turbine [8].

There are several variations of the basic method, each claiming its own advantages.

Single stage ROT: in this configuration a single row of blades at the circumference of the rotor disc is used. A fixed nozzle arrangement directs the vapour to the blades. The single stage ROT can be accomplished with a relatively small rotor disc size. The expansion ratio will depend on the ratio of the disc diameter to inlet port diameter.

Multistage ROT: in this configuration several rows of blades are used on the rotor, with intermediate blades on the stator portion of the turbine expansion section. The blade size increases from the centre to the outside of the rotor disc, giving the same effect as an axial turbine. Fig. 10 shows a typical example. A larger rotor disc is required than for the single stage ROT, but efficiency is claimed to be higher. The multistage ROT requires more machining during manufacture and needs closer tolerances than the single stage ROT.

Multiple stages allows greater energy extraction than a single stage. Several manufacturers offer versions of the multistage ROT, using different blade and vane configurations.

Fig. 10: Multistage radial outflow turbine.

Screw and scroll expanders work on the positive displacement principle where a quantity of the working fluid is trapped within the expander mechanism and expands as the mechanism rotates. Expansion of the fluid causes the rotation. These expanders are essentially positive displacement compressors operating in reverse.

Screw expander

50 to 150 kW

The screw expander or is a screw compressor operated in reverse mode. Several adaptions and improvements have been made to optimise the expander mode of operation. The screw-type engine is a displacement rotary engine. Similar to piston engines, displacement-type engines are characterised by a closed working chamber. The volume of the working chamber changes cyclically, which leads to a decrease of the energy content of the fluid in the chamber. The main parts of a screw-type engine are the male rotor, the female rotor and a casing, which together form a V-shaped working chamber whose volume depends solely on the angle of rotation (Fig. 11).

Fig. 11: Expansion cycle of screw expander.

The vapour enters the casing through the intake port in the passage formed between the tips of the rotor teeth. During rotation the volume of the chamber increases. Intake is finished when the rotor faces pass the guiding edges and the chamber is separated from the intake port. At this stage steam expansion starts and mechanical power is produced at the output shaft. During expansion the volume of the chamber continues to increase, whereas the energy content of the fluid decreases.

This process continues until the exhaust process starts and the steam is extruded. It leaves the machine through the exhaust port. How often this process takes place during one rotation of the male rotor depends on the number of teeth on the male rotor.

A significant advantage of the twin screw expander is the ability to operate in two phase, or “wet” conditions. This two phase operation means that the refrigerant does not have to be 100% vapour. The ability to operate in a range of working fluid conditions from superheat to wet vapour allows the unit to follow variable heat loads and produce power over a wide range of input conditions.

Scroll expander

5 to 50 kW

Very popular on the market for many years, scroll compressors are known for their efficiency and robustness for various applications such as ventilation or refrigeration. The scroll compressor can be used in reverse mode as an expander, and there are several companies producing units designed for power generation. Physical size limits the range of these products to 50 kW output.

In a scroll expansion is achieved by relative contact between two spiral curves. As one curve moves relative to the other, multiple crescent-shaped pockets are formed that decrease in size as one curve translates along a closed continuous path (Fig. 12).

Fig. 12: Expansion cycles on a scroll expander [5].

Expanders used in the organic Rankine cycle can also be used with low temperature “wet” steam applications.

Commercially available systems

There are fully developed systems available from a number of different manufacturers, covering the full range of heat sources and working liquids. Smaller units in the power range <50 kW are available as a complete assembled and transportable module, containing evaporator, turbine, alternator, and condenser mounted together in a transportable framework (Fig. 13). The alternator or generator is generally integrated with the turbine or expander to form a complete unit.